The present invention relates to an improved light weight solar energy concentrator and in one aspect to an improved thin flexible Fresnel-type lens for focusing solar radiation incident on the lens' outer surface onto a target area by refraction.
The concept of utilizing solar energy is not new. One of the earliest references in literature to the use of solar energy is made in Aristophanes' "The Comedy of the Clouds", which was performed in 434 B.C. In this play, one of the characters, Strepsiades, declares that he will destroy a was tablet record of a debt by using the sun and a glass lens to melt away the writing. In order for this remark to be appreciated by the theater-going public, it must have been common knowledge that the rays of the sun could be focused to generate heat. In addition, Lactantius, in 303 A.D. stated that a glass globe filled with water and held in the sun would start a fire on even the coldest day. Further, an entry in the inventory of the vestry of West Minster Abbey dated 1388 records the kindling of the "new fire" on Easter Eve with a "burning glass", and in 1745, the French scientist, Buffon, conducted experiments in generating high temperatures by concentrating the suns's rays.
Solar energy is especially attractive today, in this age of diminishing fossil fuels in which the public's awareness of atmospheric pollution and apprehension of nuclear energy has encouraged the development of alternative energy sources. Solar energy, as such an alternative, is inexhaustible and pollution free. A few of the present applications of solar energy that have evolved are the generation of electricity with photovoltaic cells, as illustrated in U.S. Pat. No. 4,204,881, distillation of water, as illustrated in U.S. Pat. No. 4,270,981, "cool" lighting for buildings, as illustrated in U.S. Pat. No. 4,297,000, and the accumulation and storage of heat, as illustrated in the below referenced patents. However, several problems have been encountered in attempting to devise an efficient, economical and practical means of concentrating this abundant enery source.
Several systems have been devised or proposed over the years in an attempt to concentrate or collect solar energy as a practical alternative to other forms of energy. Generally, there have been three types of solar concentrators or collectors proposed, namely, those employing mirrors, lenses, or a combination of both.
For example, one such system has utilized concave, parabolic mirrors in the form of a large dish, as illustrated in U.S. Pat. No. 4,111,184. In addition, other systems have utilized reflective troughs, as illustrated in U.S. Pat. Nos. 4,385,430 and 4,211,211, or an array of concentric annular conic frusta, as illustrated in U.S. Pat. No. 4,347,834. Further, a system has used flat mirrored surfaces as illustrated in U.S. Pat. No. 4,344,417 as well as flat fresnel mirrors, as illustrated in U.S. Pat. No. 3,058,394.
The reflective trough appeared to offer the best potential for high concentration of solar energy. However, this goal was seldom, if ever, achieved because of the precision required in the curvature of the reflective surface where any angular change at any point from the prescribed surface resulted in a two fold deviation of the reflected solar radiation. In addition, vibration of the reflecting surface could not be tolerated. The need to maintain the accuracy and steadiness of the reflecting surface over long periods of time required that the reflector be constructed of very rigid materials and be carefully aligned. Thus, a heavy and massive supporting structure was required. In addition, this massive structure had to be mounted so that it would remain vibration-free while tracking the sun in diurnal motion. Further, such structures have not been proved adaptable or feasible for use in space where weight reduction is of monumental importance.
In an attempt to overcome some of the limitations of the above reflective structures, refractive lenses have been utilized. Examples of Fresnel-type lenses used to concentrate solar energy are illustrated in U.S. Pat. Nos. 4,289,118; 4,194,949; and 4,011,857. In addition to the above, such lenses have also been utilized in the form of flat Fresnel lenses, as illustrated in U.S. Pat. Nos. 3,985,118 and 3,203,306, convex Fresnel lenses, as illustrated in U.S. Pat. No. 4,116,223, semi-cylindrical or tubular, as illustrated in U.S. Pat. Nos. 4,299,201 and 3,125,091, or a linear array of refracting Fresnel elements, as illustrated in U.S. Pat. No. 4,069,812.
Several limitations have been associated with the utilization of such lenses. In situations where flat Fresnel lenses have been used, they have resulted in limited apertures because of excessive chromatic aberration near the edges of the lenses. Arcuate shaped lenses have attempted to overcome this limitation but because of the need to maintain the desired configuration they have had to be constructed of a rigid material or required a complex support system. In any event, the above lens structure, like the mirror structures, has not proved adaptable or feasible for use in space where light weight construction and ease of deployment without precision alignment are of monumental importance.
Lastly, a combination of refracting lenses and reflecting mirrors have been utilized to concentrate solar energy in an attempt to overcome the above limitations, an example of which are illustrated in U.S. Pat. Nos. 4,337,759; 4,238,246; and 4,022,186. Additionally, a refracting lens and reflecting trough combination have been utilized, as illustrated in U.S. Pat. Nos. 4,323,052 and 4,230,094. However, these structures are rigid in an attempt to maintain their configuration or shape.
The present invention affords an improved light weight refracting solar energy concentrator and thin flexible Fresnel-type lens which achieves and maintains high operational efficiencies with minimal weight and substantially reduced manufacturing cost. In addition, because of the structures simplicity of construction, it can easily be deployed in space. Further, notwithstanding such simplicity of construction and its compactness, distortion of the refracting surface will not materially affect the efficiency of the concentrator, rendering the concentrator particularly advantageous for use in space.